The promise of semiconductor based radiation detectors has not been fully met despite high demand for such detectors in many areas, including homeland security, medical imaging, spectroscopy and astrophysics, because the most promising semiconductor materials for this application do not produce high crystal growth yields while those materials that produce high yields do not possess ideal material properties. (Balboa, O. S., Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals, Doctoral Disertation, Vanderbilt University (2009))
Semiconductor radiation detectors are the most recent type of detectors developed, and they are very useful because they convert X-ray and gamma quantum (photons) directly into electrical pulses and they have small dimensions. The main requirements for a semiconductor crystal based detector are: (1) a large atomic number to absorb the radiation, (2) a wide energy bandgap (Eg,eV) of the semiconductor to work at room temperature, (3) high resistivity, (4) good electron transport properties, (5) homogeneity, and (6) crystalline perfection. (Toney et al., Nucl. Instrum. Methods 1999; A428:14) The last four properties are highly dependent upon the crystal employed as the semiconductor and the technology of growing of such crystals. For example, the lifetime of electrons and holes before their recombination (the most important value of the detector specification) usually changes in the region 10−12 sec to 10−2 sec and may be improved upon by as much as thousands of times by virtue of increasing the purity of the crystal material and lowering the concentration of crystal defects. Crystal quality can be improved by those skilled in the art by improving growth conditions and material purity. Improvement of crystal purity and quality creates a better signal by limiting premature electron hole recombination and trapping. Premature recombination of electrons and holes occurs at the location of defects and impurities in the crystal lattice. Premature recombination means recombination before the electrons and holes reach the positively and negatively charged electrodes that are attached to the crystal to form the detector. Trapping is the immobilization of a hole or electron near the middle of the band gap caused by crystal impurities occupying substitution lattice positions. (Balboa, O. S., Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals, Doctoral Disertation, Vanderbilt University (2009))
In its simple form, a semiconductor has a valence band and a conduction band which are separated by the bandgap of forbidden energies. At 0° K the valence band is completely full of electrons, and the conduction band is completely devoid of electrons. As the temperature rises more electrons inherent in the material migrate from the valence band to the conduction band creating noise current in a semiconductor detector. The larger the bandgap between the conduction band and the valence band, the less noise that occurs at the same temperature. It is known that a bandgap range of 1.7 eV to 2.2 eV is the ideal value for room temperature radiation detector performance. (Toney et al., Nucl. Instrum. Methods 1999; A428:14; Glemen F. Knoll, Radiation Detection and Measurement, 3rd ed., John Wiley & Sons, pp. 353-357)
Heat is not the only way to energize an electron. The absorption of radiation or collision with an energetic charged particle (proton or quantum) produces the same effect. Once an electron has crossed over to the conduction band, it will move under the influence of an electronic field. The electron leaves a vacancy (known as a “hole”) in the valence band. The combination of the two is known as an electron hole pair. The hole, which represents a positive charge, can also be made to move in an electronic field but in the opposite direction of the electron. The motion of both contributes to the observable conductivity of the material. The configuration lasts a short time (10−12 seconds), and in the absence of an electronic field, the electrons and holes recombine and the semiconductor crystal returns to its neutral state. Nicholas Tsoulfanidis and Sheldon Landsberger, Measurement and Detection of Radiation 3RD Ed., 2011, p. 192.
A detector is normally a part of a detection system having a semiconductor crystal with electrodes deposited on its surface and situated within an electrical field. For example, the crystal may have electronically biased (cathode and anode) electrodes. The signal may be conducted to a preamplifier producing a voltage pulse with an amplitude (height) proportional to the energy of the incoming photon, then to a shaping amplifier that amplifies the signal and converts the signal to a Gaussian pulse, followed by a multi channel analyzer that generates a spectrum of the incoming proton. (Glemen F. Knoll, Radiation Detection and Measurement, 3rd ed., John Wiley & Sons, pp. 592, 610, 627, 665-680.)
Detectors may be constructed in many different configurations, such as planar (FIG. 10, 11, 3, 14), co-axial (FIG. 15), pixilated (FIG. 14) and Frisch-ring and other insulated and partially insulated detectors. The common feature of all of these detectors is a semiconductor and electrical contact means, although in Frisch ring detectors there is a non-contacting electrode as there exists a thin layer of insulation between all or part of it and the crystal. For example, a planar detector as shown in FIG. 13 is constructed from a Cd1-xMgxTe crystal with gold contacts applied to its surface. Typical detector contacts are gold, platinum, copper and aluminum. The contacts may be deposited by various means, but a common technique for applying gold and platinum contacts is electroless metal deposition using solutions of AuCl3 or PtCl4. The solution creates a chemical reaction with the surface of the crystal which deposits the film on the crystal. Detectors may also be used in arrays, such as is an x-ray imaging system. There are also surface preparation requirements to construct the detector, the most prominent of which is etching of the surface of the semiconductor to eliminate surface stress due to the process of grinding and polishing the crystal material and to improve surface perfection. Stress creates defects on the polished surface of crystals which may be the source of recombination of electrons and holes, trapping centers and also a source of reduced resistivity at the perimeter which could create current noise. Etching removes that disturbed layer on the surface of the crystal. Etching may be done with a Bromine-methanol solution. (Balboa, O. S., Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals, Doctoral Dissertation, Vanderbilt University (2009))
Germanium and Silicon have been widely used as radiation detectors because high purity perfect crystals can be grown in large volume. However, a germanium detector has a small energy bandgap (0.67 eV), and this disadvantage requires that the detector be continuously cooled, making it useless at room temperature and limiting its applicability to portable detector devices. Silicon, with with a low atomic number (14) and an energy bandgap of 1.1 eV, cannot be used for energy higher than a few tens of keV. (Balboa, O. S., Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals, Doctoral Dissertation, Vanderbilt University (2009)) A bandgap in the range of 1.7 eV to 2.2 eV is considered the ideal value for room temperature radiation detector performance. (Toney et al., Nucl. Instrum. Methods 1999; A428:14)
HgI2 has an energy bandgap 2.13 eV and therefore can operate at room temperature, but it suffers from several shortcomings including a surface that becomes degraded after use for a short time making it impractical to employ as a commercial detector. (Glemen F. Knoll, Radiation Detection and Measurement, John Wiley & Sons 2000, p. 484-486) Recently developed CdMnTe has a bandgap of 1.57 eV and has promise for detector applications but it still suffers from very poor crystalline perfection. (Hossain et al., Journal of Electronic Materials, 2009; 38(8): p. 1593-1599)
Cd1-xZnxTe (also known as “CZT”) detectors for X- and gamma rays are produced commercially by several companies including Radiation Montitoring Devices, Inc. and the E.I. Detection Imaging Systems Division of Endicott Interconnect Technologies, Inc. This material is a good radiation absorber because its constituents have large atomic numbers (48, 30, and 52). The energy bandgap of Cd1-xZnxTe increases about 6.7 meV per atomic percent of Zn from 1.5 eV of CdTe. However, the widespread deployment of CZT detectors is impeded by high-cost yields due to limited high-quality and large-volume single crystals. (Glemen F. Knoll, Radiation Detection and Measurement, John Wiley &Sons 2000, p. 486-488) These crystals suffer from twins, grain boundaries, Te inclusions, and a high density of dislocations. (Balboa, O. S., Surface and Bulk Defects in Cadmium Zinc Telluride and Cadmium Manganese Telluride Crystals, Doctoral Dissertation, Vanderbilt University (2009)) These disadvantages are pre-determined by the mismatch of the lattice constant of CdTe and ZnTe (0.648 nm and 0.610 nm) and the high segregation coefficient of Zn in CdTe (1.35), both of which tend to create crystal defects such as twins and in-homogeneities. Although large Cd1-xZnx Te crystals are grown, only a small portion of the ingot has perfect properties for X- and gamma ray spectroscopy. The typical composition Cd1-xZnxTe based detectors is Cd0.9Zn0.1Te with an energy bandgap 1.57 eV. Although a higher concentration of Zn would produce a bigger bandgap closer to the 1.7 eV ideal, crystal growth technical problems have made higher Zn concentration unachievable.
There is a commercial need for a semiconductor detector of X- and gamma rays produced from high yield large size semiconductor solid solution crystals that produce a high energy bandgap suitable for use at room temperature.